Products made from paper webs such as bath tissues, facial tissues, paper towels, industrial wipers, food service wipers, napkins, medical pads and other similar products are designed to include several important properties. For example, the product should have a relatively soft feel and, for most applications, should be highly absorbent. High bulk is also often preferred in such products. For example, three dimensional, high bulk paper products are often preferred over thinner, more two-dimensional products.
Several methods have been proposed in the past for imparting three-dimensional structures to a fibrous paper web. One well-known method is embossing, wherein the fibers in the web are mechanically deformed under high mechanical pressure to impart kinks and microcompressions in the fibers that remain substantially permanent while the web is dry. When wetted, however, the fibers may swell and straighten as the local stresses associated with the kinks or microcompressions in the fiber relax. Thus, embossed tissue when wetted tends to lose much of the added bulk imparted by embossing, and tends to collapse back to a relatively flat state. Similar considerations apply to the fine texture imparted to tissue by creping or microstraining, for such texture is generally due to local kinks and microcompressions in the fibers that may be relaxed when the tissue is wetted, causing the tissue to collapse toward a flatter state than it was in while dry.
Other methods are known in the art for protecting the strength of a paper web, such as when the paper web is wet. These methods, however, do little to protect the texture or added bulk of the web while maintaining web strength. For example, wet strength agents may be used in tissue and other paper webs to help strengthen or protect fiber-fiber bonds of the web as it dries, but such agents do not protect additional texture imparted to the dry web by embossing, creping, microstraining, or similar processes. When an embossed web which has been treated with wet strength agents is wetted, the swelling of the fibers and/or the relaxation of stresses in the fibers tends to remove much of the embossed texture as the web returns to the topography that existed as the web initially dried when the wet strength agents became activated or cured.
Thus, there is a need for a method of converting a dry tissue web or other porous web into a structure having enhanced texture and physical properties. Moreover, there is a need for a highly textured web which may maintain a high level of added bulk even after becoming wet.
Further, wet-resilient webs, such as those treated with a wet-strength agent, tend to have substantially uniform physical properties in the web. Physical properties of a paper web could be improved through a more heterogeneous structure. Thus, there is a further need for a high bulk fibrous web having heterogeneous physical properties and an improved method for producing such a heterogeneous web.